Repeatedly throughout Earth's history, giant pools of magma greater than 100 cubic miles in volume have formed a few miles below the surface.
They are the sources of super-eruptions - gigantic volcanic outbursts that throw 100 times more superheated gas, ash and rock into the atmosphere than run-of-the-mill eruptions, enough to blanket continents and plunge the globe into decades-long volcanic winters.
The most recent super-eruption took place about 27,000 years ago in New Zealand, well before humans kept records of volcanic eruptions and their aftermath. Geologists today are studying deposits from past super-eruptions to try and understand where and how rapidly these magma bodies develop and what causes them to eventually erupt. Despite considerable study, geologists are still debating how quickly these magma pools can be activated and erupted, with estimates ranging from millions to hundreds of years.
Now a team of geologists have developed a new "geospeedometer" that they argue can help resolve this controversy by providing direct measurements of how long the most explosive types of magma existed as melt-rich bodies of crystal-poor magma before they erupted. They have applied their new technique to two super-eruption sites and a pair of very large eruptions and found that it took them no more than 500 years to move from formation to eruption.